Where does our solar system end and
interstellar space begin? The Sun and its planets are
embedded in the Milky Way galaxy. The Sun's domain is known
as the heliosphere, a bubble that the Sun has blown into
the interstellar medium. In the heliosphere the Sun reigns
supreme: almost all of the particles come from the Sun, and
its emissions have huge effects on the magnetic
environments of the planets. A solar wind breezes through
the solar system at about one million miles an hour,
transporting particles and magnetic fields.

Our Special Topics analysis of papers on planetary
exploration, published 2001–2011, has identified
Professor Stamatios (Tom) Krimigis as a highly cited author
in the field of heliospheric physics. He has contributed to
112 papers that have received 1,605 total citations in the
past decade, which places him at #9 by total papers and #16
by total cites in the analysis. Two of these papers appear
on the top 20 lists in the analysis as well.

Krimigis is the Emeritus Head of the Space Department
at the Johns Hopkins University Applied Physics Laboratory
in Laurel, Maryland. He also occupies the Chair of Science
of Space at the Academy of Athens in Greece.

In this interview, ScienceWatch.com astronomy
correspondent Simon Mitton talks with him about his work in heliospheric
physics.

You have completed over four decades of
research in space physics, and in that time you've been involved in
instrumentation for all eight planets, from Mercury to Neptune, and even
to Pluto, now a nanoplanet. I don't know of any other space scientist
who can make that claim. Tell me, what attracted you to space research
in the first place?

I started at the University of Minnesota as a freshman majoring in physics
in the same month that Sputnik 1 was launched, October 1957. Needless to
say, that feat impressed my fellow students and me no end. There was a lot
of excitement on the campus at the time. The "Space Age" had begun!

The reaction of the general public in the US
verged on panic, and Sputnik became an icon of the Cold War. In 1958
however, President Eisenhower signed the legislation to create NASA. How
did you get your early hands-on experience of space
instrumentation?

Within about a year and a half I was working at Minnesota with Professor
John Winckler, who was flying balloons at high altitudes to look for
so-called solar cosmic rays, or high-energy protons from the Sun. I built
ion chambers: the detectors used to discriminate between ions and
electrons. This got me involved in looking at the sky.

Later, after James Van Allen had discovered the radiation belts, the
physics students at Minnesota invited him to give a talk. While on a tour
of Winckler's lab, Van Allen asked me how the ion chamber worked. I
explained it to him, following which he asked me if I had considered
graduate school. As a matter of fact I had but I was unsure if I had enough
funds. He said "Why don't you apply to Iowa?" And that's how I came to
leave Minnesota and go to Iowa to work with Van Allen.

What a lucky break! Did you get to work on
the radiation belts?

Yes. In my second year with Van Allen he said "How would you like to build
a solid state detector to fly on Mariner 3 and 4, the first fly-by
missions to Mars?" The big question in 1964 was: is the Earth's radiation
belt unique? I worked on the spacecraft instrumentation to address this
question.

“Krimigis standing by the Voyager proof-test model
spacecraft on display at the Jet Propulsion Laboratory. The LECP (Low
Energy Charged Particle) instrument for which he is Principal Investigator
can be seen above his left shoulder.”

After you completed your doctorate you moved
in 1968 to Johns Hopkins where the Applied Physics Laboratory was
looking for a person to lead their space physics and instrumentation
effort. How did you start on this task?

I soon put together a team of youngsters, and we proposed to design
innovative spacecraft instruments that NASA eventually selected for
Voyagers 1 and 2. Their primary mission was to explore Jupiter and
Saturn, but on the way to the giant planets they probed the interplanetary
medium.

For the Voyager missions, we designed our instruments with an eye
to cosmic rays, low energy particles, and the extended solar atmosphere, or
solar wind, which we knew went past Earth and Mars. Everyone wanted to know
how far the solar wind extended? The feeling was that it could not reach
any further than Jupiter, beyond which you would only have more or less
galactic material.

Two of your most-cited papers are about
Voyager 1 (V1) and the heliosheath, the outermost layer of the
heliosphere where the solar wind is slowed by interstellar gas. There
seems to be a fascination with the concept of a spacecraft leaving the
solar system.

Today the history of those flights is well known: we went past Jupiter,
then Saturn, past Uranus and Neptune, and yet the solar wind kept flowing.
It was a cat-and-mouse game with the theorists always predicting that the
Voyagers would reach the edge in another two years.

In August 2002 V1 approached the termination foreshock at about 85
astronomical units, 13 billion kms from the Sun. That's when the
Voyager team published the first paper (Krimigis SM, et
al., "Voyager 1 exited the solar wind at a distance of similar to 85
AU from the Sun," Nature 426[6962]: 45-8, 6 November 2003). At
that stage we knew we were close, in the so-called Foreshock region, but as
it turned out, we had not actually crossed.

Finally, by the end of 2004 V1 really did reach a zone at 14 billion kms
where the solar wind velocity dropped from 1.5M km/hr to 0.5M km /hr, but
it had not become subsonic, which was not what our models were saying. We
had however arrived at the termination shock, and that's covered in my
most-cited paper (Decker RB, et al., "Voyager 1 in the foreshock,
termination shock, and heliosheath," Science 309[5743]: 2020-4, 23
September 2005).

Voyager2 (V2) was
on a different trajectory. Did it get the same result?

That aspect is covered in my 2008 Nature paper with Robert Decker
as the lead author (Decker RB, et al., "Mediation of the solar
wind termination shock by non-thermal ions," Nature 454[7200]:
67-70, 3 July 2008). We delineated the crossing by V2 at 13 billion km.
Things then became somewhat more complicated: the envelope of the solar
wind was not really shaped like a sphere—the north part extended
further out than the south.

That gave rise to a lot of modeling activity by the heliophysics community
and that's why the number of citations continues to increase. The modeling
is still in difficult straits and I can tell you we have seen the radial
component of the solar wind drop to zero within the last year (Krimigis SM,
et al., "Zero outward flow velocity for plasma in a heliosheath
transition layer," Nature 474, 359-361, 16 June, 2011). We think
we will reach interstellar space within a year or two, but Voyager has
proved us wrong before!

I sense there is a tension between the
theorists, or model makers, and the scientists such as yourself who are
interpreting the data.

Krimigis and team for the MIMI (Magnetospheric Imaging Instrument)
experiment on the Cassini spacecraft now orbiting Saturn at a recent
(4/30/11) meeting at the Johns Hopkins Applied Physics Laboratory.

The intense scientific interest in these papers arises because the V1 and
V2 data keep disproving the models, so theorists are less than pleased! But
our observations of the velocity are made in the ecliptic plane. In order
to get the north-south component, I proposed that we rotate the spacecraft
70°. Everyone was enthusiastic. A test two months ago was successful,
so now we look at the north-south flow a few days each month, which is new.

These spacecraft have been out there for 34 years and we are still able to
make them do gymnastics. I have no doubt that until we run out of power
sometime after 2020, the Voyager Interstellar Mission is going to keep
surprising us.

Other highly cited papers of yours take us to
Saturn, where the magnetosphere imaging instrument (MIMI) on the Cassini
mission is generating many citations.

Cassini has been in orbit at Saturn since 2004. MIMI has several
components, but the one that has been rather seminal is an instrument that
can image the magnetosphere as a whole. The 2004 Space Science
Reviews paper (Krimigis SM, et al., "Magnetosphere imaging
instrument (MIMI) on the Cassini mission to Saturn/Titan," 114[1-4]:
233-329, 2004) describes the instrumentation and the anticipated science.
The package produced a wonderful harvest of astounding observations, such
as those detailed in the 2005 Science paper, "Dynamics of Saturn's
magnetosphere from MIMI during Cassini's orbital insertion," (Krimigis SM,
et al., 307[5713]:1270-3, 25 February 2005).

We are now in a position to make movies of the motion of plasma within the
magnetic environment of Saturn. We see the generation of virtual explosions
of ions accelerated near midnight at Saturn—they rotate like a
merry-go-round at the period of Saturn's rotation. This is an example of
absolutely new material.

Another example of new material generated by
MIMI is the 2008 Geophysical Research Letters paper (Cravens
TE, et al., "Energetic ion precipitation at Titan," 35[3]: art.
no. L03103, 6 February 2008). Your team identified energetic protons and
ions in Saturn's outer magnetosphere, and these can enter Titan's
atmosphere.

Cravens and the rest of us looked at what a bombardment of protons and
electrons from the magnetosphere would do to Titan's ionosphere. We found
out that Titan's upper atmosphere is not just influenced by solar
radiation. Rather, a major component is influenced by ionization by the
incoming flux of particles from the magnetosphere of the planet. In this
paper Tom Cravens did a super job of putting all the data together and
presenting a great model of Titan's ionosphere.

Finally, I was intrigued to see that the
second most-cited paper, which dates from 2001, concerns the MESSENGER
mission to Mercury (Solomon SC, et al., "The MESSENGER mission
to Mercury: scientific objectives and implementation," Planet. Space
Sci. 49[14-15]: 1445-65, December 2001). MESSENGER was successfully
inserted into orbit on March 18, 2011. Presumably the citation rate is
now speeding up because MESSENGER is at Mercury and has commenced the
science mission.

Absolutely! I'm deeply involved and I lead the atmospheres magnetospheres
discipline group as a MESSENGER co-investigator. Mercury is my eighth
planet! This paper describes what we were hoping we would see once we got
to Mercury. Some of the predictions we made 10 years ago are fairly close
to the mark but I can tell you "you ain't seen nothing yet!"

The citation count certainly draws attention
to keen interest in the planetary science community. I hope this will be
a highly successful mission, well worth the 10 years you have
waited.